I use fitteds and I've noticed that not all of my nighttime diapers smell, only the ones with fleece. Goodmama Goodnights are the best for my heavy wetter but they tend to smell, SBISH OBFs are better with smells but don't hold up for that long. Twinkie Tush Night Night is a winner here - it holds a lot of pee and doesn't smell but they are huuuuge and I am afraid that DS is uncomfortable.
So I use GMs and keep stripping them every now and then with Dawn, and I use Grovia Mighty Bubbles every 3 washes, and I wash the diapers on 'Sanitize' cycle. It helps but for a short time only.
I've heard that boiling the diapers can help a lot